The term "microwave assisted chemistry" refers to the use of electromagnetic radiation within the microwave frequencies to provide the energy required to initiate, drive, or accelerate certain chemical reactions. As chemists have long been aware, the application of heat energy is one of the most significant factors in increasing the rate of a wide variety of chemical reactions. Thus, generally familiar devices such as the Bunsen burner, other types of gas burners, hot plates, and other similar devices have historically been used to initiate or accelerate various chemical reactions.
As a relatively crude comparison, microwave assisted chemistry techniques are used to heat chemical reagents in the same way that a consumer microwave oven cooks food. There are significant differences, however, between the ordinary consumer use of microwave energy with food and its laboratory use with chemical reagents. Thus, the devices and techniques required for microwave assisted chemistry are generally much more sophisticated than are the consumer-oriented devices and techniques.
In one comparison, however, a laboratory microwave device and a consumer microwave offer the same advantage: in many circumstances they both greatly increase the rate at which materials can be heated as compared to the rates that they could be heated by ordinary conduction or convection heating. Thus, microwave assisted chemistry has been particularly valuable in driving or accelerating reactions that tend to be time-consuming under more conventional heating techniques. Particular examples include moisture analysis, in which samples must effectively be heated to dryness; digestion, a process in which a chemical composition is broken down into its elements for further analysis, with the breakdown generally being accomplished by heating the composition in one or more mineral acids; and the Kjeldahl techniques for nitrogen determination. Using conventional heating techniques, moisture analysis, Kjeldahl, or digestion reactions can be very lengthy, extending for hours in some cases. When the reactions are microwave assisted, however, they can be completed in a much shorter period of time. It will be understood that this time savings has a particularly significant advantage in any situation in which large number of samples must be tested on an almost continuous basis. Thus, although microwave assisted chemistry is relatively new compared to some other techniques, it has become well established and accepted in a number of analytical applications.
As well understood by those familiar with the electromagnetic spectrum, the term "microwave" is often used generically to refer to radiation with wavelengths of between about 1000 and 500,000 microns (.mu.), and corresponding frequencies of between about 1.times.10.sup.9 and 5.times.10.sup.11 Hertz (Hz). These are arbitrary boundaries, however, and other sources refer to microwaves as having frequencies of between about 10.sup.8 Hz and 10.sup.12 Hz and wavelengths of between about 300 centimeters (cm) and 0.3 millimeters (mm). For commercial and consumer purposes in the United States, the available microwave frequencies are regulated by the Federal Communications Commission and are generally limited to certain frequencies such as 2450 megahertz (MHz). Because of the relatively long wavelength of microwave radiation, microwave assisted chemistry techniques are often carried out in closed vessels which are in turn placed inside a device that bears a superficial relation to a consumer microwave oven, but that is much more sophisticated in its source, waveguide, cavity, and control elements.
In turn, because the reactions are often carried out inside closed vessels, and because the reactions often generate gas, the reactions tend to generate and build up significant pressure in the reaction vessels. Accordingly, vessels have been developed to withstand most expected pressures, and also to include various pressure relief devices to prevent the vessels from exploding under the significant pressures being generated. An exemplary vessel and pressure release system is set forth, for example in U.S. Pat. No. 5,369,034, which is assigned to CEM Corporation of Matthews, N.C.
In many of these existing vessels systems, however, the pressure release function destroys or consumes, even if intentionally, a part of the vessel system (e.g., a rupture disc). Thus, even though such parts are intended to be easily replaced, doing so can represent a disadvantage in certain circumstances.
Accordingly, in more recent attempts at solving the problem, vessels have been designed in which the venting mechanism is more permanent. One example is U.S. Pat. No. 5,270,010 to Lautenschlager. In this device, a domed spring with a particular structure is used to help hold the lid on a pressure vessel for microwave assisted chemistry. It has been found in actual practice, however, that the performance of the spring degrades over time, particularly under the high pressures experienced by these vessels. Thus, although the domed spring does not need to be replaced every time the vessel vents gases, it does have to be replaced on a regular basis.